Steve Lewin has given you the straight poop on breaking in any ABC or ABN engine, Never make blubbery rich initial runs. Most bronze sleeves are machined with a choked taper bore and this sleeve expands more than the aluminum piston with heat therefore to acheve the proper piston/sleeve seal at operating temperature the cold fit at the top of the bore is squeaky tight and at running temp you will have the right clearance. If you break in rich therefore cold there will not be enough clearance and the sleeve will be damaged and never able to perform as intended.

By the way the Magnum .28's are true ABC and are excellent little engines with great carbs that gives the engine a very respnsive throttle when tweaked right. There are four of them on the airplane in my avitar.

John